Huge performance drop for fsc.exe/dll #74286
Description
Description
Tracking issue to cover dotnet/fsharp#13730
Between .NET 7 preview 2 and preview 3 the performance of the F# compiler (fsc.dll/fsc.exe) on large inputs dropped by about 8x (yes, 800% regression). This stems from region-based GC being enabled by default.
Details are above. Particular characteristics are
- Large application liking to allocate 1GB+ of memory
- Not Server mode GC
- GC Batch mode enabled
- Large sections have performance dominated by a single thread of execution (since large sections are not parallelized)
Previously .NET Framework and .NET Core GC has performed well with this application
NOTE: This was only noticed when previews 5 and preview 7 were inserted back to dotnet/fsharp and the F# compiler bootstrap was executed. Catching this much earlier would have been possible, e.g. via a .NET performance test that runs the latest inserted F# compiler on latest runtime.
Reproduction Steps
See repro steps in dotnet/fsharp#13730 for the particular case where the compiler is used to compile the F# compiler. We expect the same performance problems on any large inputs to the F# compiler.
Expected behavior
See repro steps in dotnet/fsharp#13730 where you can run the compiler with various version of .NET runtime by adjusting settings. Performance of the F# compiler should match previous versions
Actual behavior
Performance has degraded by about 8x, 200+ second generation GCs where previously there was one.
Regression?
Yes
Known Workarounds
No perfect workarounds.
- Setting
DOTNET_gcServer=1helps signficantly but this still gives a 25% slowdown. - Disabling region-based GC solves the problem but can only currently be done via an environment variable
Configuration
See links above. Performance testing has only been done on Windows.
Other information
See discussion linked above